1. Field of the Invention
The present invention relates to a filter circuit, radio communication equipment and a signal processing method, and for example relates to a filter circuit for band limitation which is connected to a post stage of a power amplifier for use in a transmission unit of communication equipment using radio.
2. Related Art
Conventionally, as shown in FIG. 23, a filter circuit is configured by cascade-connecting resonators 1107(1), 1107(a), 1107(3), . . . 1107(n). An equivalent circuit of each resonator is made up of an inductor and a capacitor, and is also added with a resistor in the case of considering an effect of a loss. A resonance frequency in the case of the resonator without the resistor is given by the following:f0=(L×C)−1/2 where L and C are respectively an inductance and a capacitance of each resonator. In the filter circuit, resonators are cascade-connected, and inter-resonator coupling coefficients (m12, m23, . . . , mn-1,n in FIG. 23) expressing coupling amounts of the respective resonators and an external Q (Qe in FIG. 23) value expressing an amount of exciting the resonators in each of input and output units are appropriately determined, so that a pass frequency range and a stop band attenuation amount as the filter circuit can be determined. Reference numeral 1101 denotes an input terminal, and reference numeral 1106 denotes an output terminal. In the filter circuit where the resonators are cascade-connected, a current is propagated from one resonator to another, resulting in flow of a current with all frequency components through the resonators. Therefore, in the case of constituting the resonators by the use of a material having a limit to a value of a current passable therethrough per unit area in a superconductive state, such as a superconductor, a power handling capability of each resonator is an important parameter for allowing large power to pass through the filter circuit, and hence studies have been conducted on a method of taking measures to prevent concentration of current flow on the resonator by application of a filled circle shape or a wide line so as to improve the power handling capability. However, there is a problem in that, since the external Q value is extremely high in the superconductive resonator, the current concentration is high and a large power handling capability cannot be obtained only by devising the shape of the resonator.
Meanwhile, as shown in FIG. 24, there is a method of connecting resonators in parallel to constitute a filter circuit as a method of dispersing power to each resonator in the filter circuit to realize a filter characteristic (JP-A 2001-345601 (Kokai), JP-A 2004-96399 (Kokai)). By such a parallel configuration of the resonators, inputted power is divided to each of the resonator 1108(1), 1108(2), . . . 1108(n) so as to increase the power handling capability as a whole. The resonator is configured by constituting the resonators having different frequencies (f1, f2, . . . fn in FIG. 24) and combining the resonators in parallel such that the resonators with adjacent resonance frequencies have mutually reversed phases, to realize the filter characteristic. In the figure, “−” in “−m2” denotes reverse-phase coupling. There is a method of combining a superconductive filter with a normal conductive filter in a filter configuration formed using the above-mentioned configuration (U.S. Pat. No. 3,380,165, JP-A 11-186812 (Kokai)). In U.S. Pat. No. 3,380,165, the superconductive filter and the normal conductive filter are arranged in parallel, but there is a problem with this as follows. When large power is supplied to the input, as it is divided and inputted into the filters, the divided power is separated only into power to be reflected in and power to pass through each of the filters, and with this shape, the superconductive filter also requires a large power handling capability.
As thus described, it has hitherto been difficult to increase a power handling capability of a filter circuit using a superconductor with a steep transmission characteristic due to a characteristic of a critical current density of the superconductor.